I'm just going to go ahead and say my unpopular opinion: Bryant's number should not be retired league wide and I think it's silly that other teams beyond Lakers would retire his number.
It's tragic that he died. But that alone isn't the kind of thing that should force an entire league to stop using your number. He's also not the absolute best to play the game.
With Robinson in MLB, you have arguably the most important player in the history of the sport. With Gretzky in the NHL, he's beyond anybody that's ever played the game by a mile (if you want my opinion I'm 50/50 on retiring a number for this too).
If all it took was being a great (not the best) player in a sport who died tragically, you'd have to retire a lot of numbers league wide in various sports.
Nobody is arguing to retire 21 league wide in MLB because of Roberto Clemente tragically passing away during a goodwill mission to Nicaragua.
However in the above instance, they did rename the annual Commissioner's award in his honor, which is something the NBA could do to honor Bryant's memory.